EPR studies in the liquid-helium temperature range (4-20 degrees k) are in progress to detect in both the mammalian and bacterial phosphorylating particles possible high-energy forms of cytochromes and/or iron-sulfur systems which may be involved in the energy coupling process at the three distinct sites of oxidative phosphorylation. Model Systems bearing on the mechanism of energy conservation are also being investigated. Two of these systems include cytochrome C3-A multi-heme oxidase type hemoprotein with properties similar to mammalian cytochrome and the phosphorylating system of the obligate aerobe Azotobacter vinelandii which has respiratory components very similar to the mammalian system. Using the aerobe A. vinelandii with growth on the nuclear isotope- Fe57 (an experiment which is most readily performed with a bacterium) has established a new iron sulfur component of great intensity and of the high-potential type in the respiratory chain. This component also exists in the mammalian chain. The chain continues to increase in complexity. The role of this new component in terms of energy coupling is actively being investigated.